1. Field of the Invention
The present invention pertains generally to transmission type optics for short wavelength electromagnetic radiation, and more particularly to an achromatic lens (a lens corrected for chromatic aberration for a specified wavelength range) for focusing electromagnetic radiation in the extreme ultraviolet (EUV) and x-ray regime with wavelengths ranging from 0.02 nm to 20 nm.
2. Description of the Background Art
The past decade has seen rapid development of optical components for 0.02-20 nm electromagnetic radiations. Diffractive, refractive, and reflective methods have all been utilized to focus short wavelength electromagnetic radiation. Of these three categories, a diffractive lens implemented as a Fresnel zone plate provides the highest resolution of approximately 25 nm with wavelengths longer than 2 nm, and 100 nm at shorter wavelengths.
There are several types of Fresnel zone plates. An amplitude zone plate consists of concentric opaque rings arranged such that the radiation passing through the rings arrives at the focal point in phase, while the out-of-phase radiation is absorbed by the rings. Alternatively, a phase zone plate's rings shift the phase of the radiation instead of simply absorbing the radiation. If the rings are designed to shift the phase of radiation by π, the theoretical maximum efficiency is 40%, quadrupled from about 10% for that for an amplitude zone plate. Both types of zone plates above are called binary zone plates since the zones (rings or empty) are of approximately equal height. A blazed zone plate consists of rings of varying heights within that are designed to provide continuous phase shifting for improving focusing efficiency. An ideal blazed zone plate in theory can provide 100% efficiency.
The focal length of a Fresnel zone plate is given by ƒz=2RΔR/λ, where λ is the wavelength, R is the radius of the Fresnel zone plate, and ΔR is the width of the finest, outermost zone. The spatial resolution of a zone plate is 0.61λ/N.A.=1.22ΔR according to the Rayleigh criteria, where N.A. is the numerical aperture of the zone plate. For a given Fresnel zone plate, the focal length depends on wavelength, and it is a chromatic lens. The monochromaticity requirement for a Fresnel zone plate is Δλ/λ<1/N=(2ΔR/R), where N is the number of zones. For an example, assume a Fresnel zone plate were to be made to have a diameter of 6 mm and outer zone width of 70 nm. It would have over 21,000 zones. Its useful bandwidth would be less than 0.005%, which is difficult to achieve and generally does not efficiently use radiation produced in a typical x-ray source.
It is difficult to make refractive lenses for short wave (e.g., 0.02 nm to 20 nm) electromagnetic radiation because the index of refraction for most materials is close to unity and attenuation is relatively high. The complex index of refraction of a material is generally expressed as                                                         n              =                            ⁢                              1                -                δ                -                                  i                  ⁢                                                                           ⁢                  β                                                                                                                        =                                ⁢                                  1                  -                                                            αλ                      2                                        ⁡                                          (                                                                        f                          1                                                +                                                  if                          2                                                                    )                                                                                  ,                                                          (        1        )            where α=nare/(2π) is a constant and is determined by the atom number density na and the classical radius of the electron re, and (ƒ1+iƒ2) represents an effective number of electrons per atom. When a beam of short wavelength electromagnetic radiation transmits through a material of thickness t, its phase is advanced relative to vacuum by 2παλƒ1t, and its intensity is attenuated by exp(−4παλƒ2t). As the wavelength decreases, ƒ2 generally decreases with wavelength to the third power except near an absorption edge, while ƒ1 generally changes little except near the absorption edge. This property was used in recent years for producing transmission lenses with limited capabilities. The focal length of a refractive lens with a single spherical convex surface is ƒr=RC/(n−1); and becomes ƒr=−RC/(αλ2ƒ1) when n is substituted using Expression (1). The focal length is usually very long for single refracting surfaces. To make a refractive objective with an acceptably short focal length for focusing short wavelength electromagnetic radiation of wavelength less than 1 nm, a number M of these lenses can be stacked up within a distance small compared to ƒr to produce a compound focal length of ƒr/M. Because the focal length depends on the wavelength, a refractive lens is also chromatic. The highest resolution achieved by refractive lenses up to date is about 300 nm. In addition, the attenuation also limits the size of the field of view in a refractive lens, as the thickness of lens increases with the lens diameter.
Mirror reflective focusing optics is intrinsically achromatic, especially when the mirror has only one single reflecting surface. For a multilayer focusing mirror, a finite bandwidth is required for obtaining effective reflection but not for achromaticity. A mirror operating at grazing incidence usually has a small field of view due to various geometric aberrations. It is therefore generally not well suited for imaging applications requiring a large field of view. The best resolution obtained from a grazing incidence mirror is larger than 250 nm. Operating at normal or near normal incidence, a mirror generally has less aberration than that operating at a grazing incidence, but its field of view is generally limited. In order to increase the field of view for both grazing and normal incidence cases, two of more reflecting mirrors are required. Examples include the well-established Wolter and Schwarzschild optics. For short wavelength radiation, the Wolter optics is limited in resolution due to difficulties associated with the requirement of making highly aspherical mirror surfaces. In comparison, the normal incidence employed in the Schwarzschild optics requires a multilayer coating for achieving adequate reflectivity. The multilayer coating requirement further limits the usable wavelength to the longer wavelength range, currently above 4 nm. The best resolution achieved up to date is about 500 nm and 50 nm by a Wolter and Schwarzschild optic, respectively.
The present invention seeks to provide an achromatic lens that overcomes one or more of the above-described shortcomings.